This invention relates to a constant power supply unit of an electric heating apparatus for supplying a constant power to the electric heating apparatus regardless of the nominal value of the AC power supply voltage.
The electric heating apparatus has two nominal power supply voltages since the supply voltage supplied from an electric company is usually either 220 or 100 volts. Conventional electric heating apparatus such as a rice pot, coffee pot, etc., use a step-down transformer in order to use one of the two nominal voltages of a power supply. Nevertheless, the requirement for a transformer is not only costly but causes extra consumption of electrical power. It is practically impossible to mount the transformer inside the heating apparatus because of weight and volume constraints. Also some electric heating apparatus such as electric rice cookers that are adaptable to two nominal input voltages have two hot wires and a switch for selecting a connection of the two hot wires. When high nominal input voltage of 220 volts is supplied from an electric company, a user sets the switch so as to make a series connection of the two hot wires. When low nominal input voltage of 100 volts is supplied, the user sets the switch so as to make a parallel connection of the two hot wires or to select one of the two hot wires. The temperature distribution of the rice in the cooker is not homogenous due to the difficulty of the hot wire arrangement, thus the rice is not properly cooked. Furthermore, a fire may occur if the user sets the switch improperly.
Accordingly, a primary object of this invention is to provide a constant power supply unit for an electric heating apparatus without using expensive, heavy, and bulky transformers and without having to switch the connection between two hot wires depending on the nominal value of the AC supply voltage.
Another object of this invention is to provide a constant power supply unit which can be mounted inside the small space of an electric cooker.
A further object of the present invention is to provide a constant power supply unit of an electric heating apparatus which is safe from an electrical fire.